Recently, we worked with a client suffering from substantial downtime issues primarily due to I/O failures. They had a redundant PLC system, and though it hadn’t malfunctioned, it was at end-of-life, so they contracted us to modernize their entire plant. As part of that process, they asked whether they should continue to use a redundant PLC setup. What are the downsides of using redundancy? What factors determine whether redundancy is needed for your system?
PLC reliability has come a long way in the last few decades. In the past, PLC reliability was hit or miss, and redundancy was useful for preventing significant unplanned downtime events. However, with PLC Mean Time Between Failure (MTBF – the average expected runtime before failure occurs; a PLC’s MTBF is often multiple times longer than the PLC’s expected lifetime) now well into the hundreds of thousands of hours, a simple PLC hardware failure is becoming more and more a thing of the past. On top of that, redundancy brings multiple inherent concerns. Due to the increased equipment requirements, redundancy can cost you substantially more than a single-PLC setup. Redundancy also adds multiple layers of complexity to troubleshooting and routine maintenance for your system, which can cause costly delays in the event of downtime due to other non-redundant components. Thanks to increased PLC reliability and those issues, redundancy has become an expensive method of preventing routine downtime.
So, are we saying that redundancy is a waste of time? Absolutely not! There are situations in which PLC redundancy is crucial. We’ve got 3 questions you should consider when deciding whether redundancy is right for your system:
1. Is the PLC safety-critical?
Situational requirements will vary, but in some cases a redundant PLC system is the right choice to protect your operators, your machinery, your community, and the environment.
2. Can your maintenance team properly manage the redundant system?
As noted above, redundant systems add layers of complexity to both the routine maintenance and equipment troubleshooting of your system. If redundancy is required for safety but you’re not sure your in-house team is prepared to handle it, then you must make sure you have a qualified controls firm keeping your systems running and operators safe. If it’s not critical for safety, then you’ll need to weigh the benefits of redundancy vs. the cost of supporting it, either in-house or outsourced. Like any control system, poorly maintained redundant PLCs can create substantial downtime headaches, costing you production efficiency and money—the very reason you chose them in the first place.
3. Are there any unique factors in your situation?
There may be business or production-specific factors for your facility that make redundancy necessary. Example: You service one major customer, delivery times are relationship-critical, and every precaution against downtime is needed, so it’s worth it. Or perhaps you operate a specialty process that takes a week or more to restart after downtime, and the cost of downtime is too high to reasonably risk. Or maybe you’re a municipal water treatment plant whose customers rely on you for clean water, and extended downtime isn’t an option. Whatever your situation, the unique factors you identify need to be weighed against the questions and inherent concerns redundancy brings.
As you can see, there are several factors when considering implementing PLC redundancy. There are also creative solutions that can help you prevent downtime in more efficient ways than redundancy. For our client, they decided to do away with their redundant framework and split their single PLC system into two systems. This allowed them to operate their two process lines in parallel, so if one went down, the plant could still run at 50% capacity. Each situation is different, so there are no one-size-fits-all recommendations. The best way to determine if you need redundancy is to talk to an independent controls expert who can help you meet your goals.